An amphibious whale from the middle Eocene of Peru reveals early South Pacific dispersal of quadrupedal cetaceans

Peregocetus holotype MUSM 3580 anchors a 42.6 Ma Peruvian protocetid occurrence and preserves weight-bearing and aquatic features; locomotor capability and a westward South Atlantic dispersal route remain phylogenetic and functional interpretations.

Peregocetus pacificus is anchored by holotype MUSM 3580 in the lowest Yumaque Member of Peru at an interpreted approximately 42.6 Ma; the value is nannofossil biochronology, not a direct specimen date, genus-wide duration or global first appearance.

Lambert et al. establish Peregocetus pacificus gen. et sp. nov. from holotype MUSM 3580 and place it within the paraphyletic Protocetidae in their morphology matrix; that placement is analysis-dependent and not a direct-ancestry claim.

Peregocetus holotype MUSM 3580 is bounded to Playa Media Luna in the Pisco Basin, Peru, at 1.95 m above the base of the Yumaque Member of the Paracas Formation; the paper's westward South Atlantic route is a reconstructed dispersal hypothesis, not an observed track.

Peregocetus preserves anatomy used to reconstruct both aquatic swimming and terrestrial weight bearing, including robust limbs and hoof-like phalanges; feeding, gait, swimming performance and habitat behaviour remain inferred rather than directly observed.

Holotype MUSM 3580 preserves mandibles and most major postcranial regions, including two fused sacrals, a defined femoral head, robust limbs and hoof-like distal phalanges; the articulated presentation includes reconstructed or missing portions.